1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece, and more particularly to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. Conventionally, as apparatuses for planarizing semiconductor wafers, there have been used a self-planarizing CVD apparatus, an etching apparatus or the like, however, these apparatuses fail to fully planarize semiconductor wafers. Recently, attempts have been made to use a polishing apparatus for planarizing semiconductor wafers to a flatter finish with more ease than those conventional planarizing apparatus.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated. This process is called Chemical Mechanical polishing.
Attempts have heretofore been made to apply an elastic pad of polyurethane or the like to a workpiece holding surface of the top ring for uniformizing a pressing force applied from the top ring to the semiconductor wafer. If the pressing force applied from the top ring to the semiconductor wafer is uniformized, the semiconductor wafer is prevented from being excessively polished in a local area, and hence is polished to a highly flat finish.
The polishing apparatus is required to have such performance that the surfaces of semiconductor wafers have a highly accurate flatness. Therefore, it is preferable that the lower end surface of the top ring which holds a semiconductor wafer, and the contact surface of the polishing cloth which is held in contact with the semiconductor wafer, and hence the upper surface of the turntable to which the polishing cloth is attached, have a highly accurate flatness, and those highly accurately flat surfaces which are kept parallel to each other in cooperation with a gimbal mechanism of the top ring unit have been used in the art.
In order to prevent a polishing surface, i.e., an upper surface of the turntable from being deformed into an upwardly convex shape due to frictional heat generated in a polishing process, there has been proposed a technique in which the turntable comprises an upper plate and a lower plate which are laminated and made up of materials having different coefficient of thermal expansion. Specifically, the coefficient of thermal expansion of the upper plate is smaller than that of the lower plate, and even if temperature of the turntable is raised due to frictional heat generated in the polishing process, the upper and lower plates expand equally because there is a temperature difference between the upper plate and the lower plate, thus keeping the upper surface (the polishing surface) of the turntable flat. As a result, both of the lower end surface of the top ring and the upper surface of the turntable are kept flat, and parallelism of both surfaces is maintained in cooperation with a gimbal mechanism of the top ring unit.
Further, for solving this kind of problem, there has been proposed another technique in which the upper surface of the turntable is deformed into an upwardly convex shape due to frictional heat generated in the polishing process, and the lower end surface of the top ring (or carrier) is caused to be deformed into a concave shape opening toward the bowed turntable by evacuating air in the chamber formed in the top ring so as to conform to the bowed turntable. Thus, the upper surface of the turntable and the lower end surface of the top ring are kept parallel to each other for improving polished wafer flatness.
Efforts have been made to find an ideal polishing surface, i.e., an ideal upper surface of the turntable and/or an ideal pressing surface, i.e., an ideal lower end surface of the top ring by inventors of the present application. It is found by the inventors that the upper surface of the turntable and the lower end surface of the top ring which are not necessarily flat are desirable.